Yeah "fear of open spaces" has always been a not-entirely-accurate characterization. But it's difficult to explain exactly what agoraphobia means. Here's what Wikipedia has to say on the subject:
Agoraphobia (from Greek ἀγορά, "marketplace"; and φόβος/φοβία, -phobia) is an anxiety disorder characterized by anxiety in situations where the sufferer perceives the environment as being difficult to escape or get help. These situations include, but are not limited to, wide-open spaces, as well as uncontrollable social situations such as may be met in shopping malls, airports, and on bridges.
That's actually about the best I've seen. It's similar to claustrophobia in that the victim feels trapped, although the causes are very different.
Statistically speaking, the odds that anyone here has an actual phobia are pretty low. Real phobias are not common. We tend to use the word "phobia" too much. I am not afraid of heights, but I would be VERY afraid if I were standing on a ledge on the fiftieth floor of a skyscraper, for example. But that's an appropriate reaction to the situation. I dislike tight spaces, but I would not describe myself as claustrophobic. True, clinical phobias are irrational and all-consuming. They can ruin people's lives. It's really quite sad.